8. M.S. Koval'chenko, V. V. Dzhemelinskii, V. N. Skuratovskii, et al., '~icrohardness of some carbides at various temperatures," Poroshk. Metall., No. 8, 87-91 (1971).

9. Yu. G. Tkachenko, S. S. Ordan'yan, V. K. Yulyugin, et el., "Preparation and high- temperature antifrlctional properties of eutectic alloys of the MeIVC--MelVB2 system," Izv. Akad. Nauk SSSR, Neorg. Mater., 13, No. 8, 1414-1418 (1977).

EFFECT OF HEAT TREATMENT ON THE WEAR RESISTANCE OF HOT-PRESSED

HARD METALS UNDER ABRASIVE FRICTION CONDITIONS

D. Kh. Bronshtein, M. G. Loshak, E. S. Simkin, N. V. Tsypin, and I. A. Sveshnikov

UDC 621.762

A combination of good wear resistance and high strength is one of the basic require- ments for materials used under intense abrasion conditions. Because of this, tool tips made of VK hard metals* and composite materials based on these alloys have found extensive application in industry.

Hot-pressed hard metals are stress-relieved by annealing [i, 2], but unfortunately no information appears to be available in the literature on the wear resistance of annealed hot-pressed hard metals. From data reported in [3-5] it follows that quenching improved some physicomechanical and operating characteristics of hard metals sintered by the orthodox method. One reason why this treatment has a beneficial effect on the properties of standard hard metals is thought to be that it strengthens their cobalt phase by increasing the amount of tungsten dissolved in it. It is reasonable to assume that quenching will be even more beneficial to the properties of hot-pressed hard metals, since the solubility of tungsten carbide in the cobalt phase of these materials can be expected to be higher because of the smaller grain size of their carbide phase compared with alloys produced by conventional sintering.

At the Institute of Superhard Materials an investigation was carried out into the ef- fect of various types of heat treatment on some properties of hot-pressed hard metals. Ex- periments were carried out on prismatic, 5-mm-square x 35-mm-long specimens made of VK6 and VK8 hard metals -- the most widely used die materials. Hot pressing was performed in graphite (EG-0 electrode grade) die sets. Prepressed hard-metal mixtures were subjected to indirect heating in an LPZ-2-67 high-frequency apparatus. The hot-pressing process parameters were: temperatures 1400-1420 and 1380-1400~ and pressures 150 and 125 kgf/cm 2 for the VK6 and VK8 hard metals, respectively. Compacts were held isothermally for 3 min and then cooled at a rate of 8-10 deg C/min.

Temperature measurements were made with an OPPIR-017 optical pyrometer. Specimens were annealed in a VT-40/400 tube furnace provided with a dried hydrogen atmosphere (dew point -- 50~ The annealing of the VK6 hard-metal specimens was performed according to the following schedule: heating to a temperature of 1250~ at a rate of I0 deg C/min, holding at 1250~ for 15 min, and cooling at a rate of 5 deg C/min. The physicomechanical proper- ties of the materials were studied by standard methods.

The tungsten contents of the cobaltphases of the hard metals were determined mag- netically, by measuring their Curie points [7] with a modified UTK-2 apparatus. Wear in- tensity assessments were made, using an AI-2 machine and a method [6] developed at the Institute of Superhard Materials, by tests involving rubbing against an abrasive interlayer under a pressure of i0 kgf/cm 2. The abrasive was green silicon carbide of No. 100/80 grain

size.

*A generic designation of WC + Co hard metals; VK6 and VK8 materials, also referred to in this article, are WC + 6% Co and WC + 8% Co alloys, respectively -- Translator.

Institute of Superhard Materials, Academy of Sciences of the Ukrainian SSR. Trans- lated from Poroshkovaya Metallurgiya, No. 6(198), pp. 52-54, June, 1979. Original article

submitted July 6, 1978.

392 0038-5735/79/1806-0392507.50 �9 1979 Plenum Publishing Corporation

Annealing was found to have virtually no effect on the hardness (90.5-91.0 HRA), density (14.9-15.0 g/cmS), and coercive force (180-190 Oe) of the hot-pressed VK6 alloy specimens. However, it produced a slight increase in their ultimate strength (from 157 • 7.5 to 185 • 6.5 kgf/mm 2) and a marked increase in their wear intensity (from 1.3 to 1.7 mm/km). The small increase in strength was probably due to relief of the internal stress in the alloy, and the fall in wear resistance to a decrease in the amount of tungsten dis- solved in the cobalt phase. Indeed, analyses of the cobalt phase established that its tungsten content was less in the annealed hard-metal specimens than in the starting hot- pressed specimens.

Next, a study was made of the physicomechanical properties of hot-pressed VK8 hard- metal specimens after quenching. Their hardness and coercive force were found to be un- affected by the heat treatment and equal to 89.5-90.5 HRA and 175-185 Oe, respectively. Quenching slightly lowered their density, but increased their transverse rupture strength and impact strength, from 182 • 4.8 to 184 • 4.6 kgf/mm 2 and from 0.18 to 0.20 kgf'm/cm 2, respectively. The quenching conditions were as follows: temperature 1200~ heating rate 400 deg C/min, isothermal holding time 5 min, and cooling in oil. In this case the wear resistance of the hard-metal specimens was determined on a special stand based on a IM-553 universal twin-turret capstan lathe. A fall in wear intensity from 6.0 to 1.6 mm/km was recorded.

For wear resistance assessments specimens on which mechanical properties had been de- termined were brazed onto tool holders and assembled into six-tool packs. The packs were machined by the electric spark method and diamond-ground to ensure that all the cutting edges were on the same level. The abrasive material was Monokhovsk quarry sandstone of hardness f = i0-ii on the M. Protod'yakonov scale. This particular sandstone was chosen becaus~ it is one of the most common hard and abrasive rocks. Blocks of this material weighing 2-3 tons each were sealed with a sand-- cement solution. Thanks to the use of these blocks all the test tools operated under identical conditions, with the same relative lengths of cutting and cooling periods, and the same cutting path length [8].

The improvement in the operating performance of the hard metals brought about by quenching is a result of the strengthening of their cobalt phase by an additional quantity of dissolved tungsten during the heat treatment. The tungsten content of the cobalt after quenching was found to be 20-25 wt.% higher compared with the starting hard metals. The dissolution of tungsten in the cobalt phase in the course of quenching is more intense in hot-pressed hard metals, presumably because of their smaller tungsten carbide grain size [3, 9]. The increase in the amount of tungsten in the cobalt solution of a hard metal after quenching can be ascribed to the fact that the rate of cooling in this treatment is higher than that in the manufacture of hard metals; as a result, no tungsten is precipitated out of a cobalt solution saturated during the heating-up and isothermal holding preceding the quenching.

Thus, analysis of the experimental results obtained leads to the conclusion that the wear resistance of hot-pressed hard metals can be substantially (almost threefold) increased by subjecting them to an additional quenching operation.

LITERATURE CITED

i. G.V. Samsonov and M. S. Koval'chenko, Hot Pressing [in Russian], Kiev (1962). 2. G.A. Meerson and V. I. Shabanin, "Effect of composition and heat treatment conditions

on the physicomechanical properties of Pobedit type hard metals," Tsvetn. Met., No. 3, 77-85 (1940).

3. M.G. Loshak and L. I. Aleksandrova, Strengthening of Hard Metals [i~ Russian], Naukova Dumka, Kiev (1970).

4. Yu. N. Chepurnin and I. F. Molokhov, "Methods of improving the cutting properties of hard metals," in: Diamond-Abrasive Machining, Tr. Permsk. Politekh. Inst., No. 149, 131-135 (1974).

5. I.I. Yanovskii and E. M. Patrikeeva, "Effect of heat treatment on the quality of a hard metal," in: Problems of Increasing the Useful Life of Drilling Tools [in Russian], Novokuznetsk (1975), pp. 25-26.

6. N.V. Tsypin, Wear Resistance of Drilling Bits [in Russian], Kiev (1968).

393

7. V.I. Tumanov, E. A. Korchakova, and S. M. Elmanova, "Magnetic properties of tungsten hard alloys at elevated temperatures," Poroshk. Metall., No. 5, 87-90 (1971).

8. L.I. Aleksandrova, D. Kh. Bronshtein, L. N. Virovets, et al., "Heat treatment as a means of strengthening hard metals produced by the hot-pressing method," in: Con- structional Materlals [in Russian], Kiev (1978), pp. 66-71.

9. D. Kh. Bronshtein, E. S. Simkin, and N. V. Tsypin, "Properties of hot-pressed hard alloys based on tungsten monocarbide," Poroshk. Metall., No. 4, 98-101 (1978).

MECHANICAL PROPERTIES OF PERMEABLE MATERIALS WITH A PREARRANGED

STRUCTURE FROM CONTINUOUS METALLIC FIBERS

D. M. Karplnos, A. E. Rutkovskii, V. A. Zorin, and A. A. Ivanchuk

UDC 621.762

The object of the work described below was to determine the dependence of the mechani- cal properties of permeable fiber materials from knitted gauzes covering a wide range of porosities on factors such as diameter of the wire employed and stacking orientation of the semifinlshed products. Test specimens were produced by stacking knitted gauzes of the "lasting" type in layers and sintering the resultant stacks in a vacuum twice with inter- mediate pressing [2]. The sintering temperature was 1350~ the holding period i h in the first slntering and 2 h in the second, and the residual pressure i0-3-I0 -~ mm Hg. The re- quired specimen shape was obtained by grinding and electric spark machining. The specimens were divided into groups according to their porosity. Deviations from mean values did not exceed 2.5%. The characteristics investigated were averaged in each group.

Unidirectional and cross stacking of gauze semifinished products were employed. With unidirectional stacking the direction of the loop columns (OX) or loop rows (OY) in all layers was the same, while with cross stacking it was changed through an angle of 90 ~ for each successive layer. The arrangement of fibers in a knitted semifinished product con- sisting of a single looped wire is shown in Fig. I. KhlSN9T (Ti-stabilized 18/9) steel wires of various diameters were used.

Static Tests. Tensile strength determinations were made on an RM-103 machine. The test specimens were identical in shape and size with those used in standard tests on solid metals. They had a unidirectional arrangement of gauze layers in stacks, and load was applied to them in the direction of the loop columns, i.e., their strength was determined in the direction OX (Fig. 2). The strength of the material grew with decreasing fiber diameter, probably because thinner wires have fewer internal defects and their specific

strength is higher.

Similar specimens were tested, using a UM~-IOT machine, in tension at elevated tem- peratures (300-600~ under atmospheric conditions. Data on the strength of the material at higher temperatures under vacuum conditions are given in [i]. With rising temperature the tensile strength of the material fell in the whole range of porosities investigated, and its elongation decreased from 12-22% at 20~ to 7-12% at 600~

To determine the shear strength of the permeable fiber material (PFM) with the pre- arranged structure, specimens 5 x 15 x 60-mm in size were tested on a UMM-10 machine. A shearing load applied to a specimen of such a material at first densifies it in the fracture plane and then shears it. According to [3], the shear strength of specimens pressed from discrete fibers is practically unaffected by their fiber diameter. What is important is the properties of the fibers themselves, which are determined by the nature of their mate- rial and the conditions of their manufacture and heat treatment during sintering. During slnterlng the fibers experience loss of strength, their structure changes, and the shear strength falls. Curves of the shear strength Of the material investigated as a function of its porosity and slnterlng temperature are shown in Fig. 3. The strength was found to

Institute of Materials Science, Academy of Sciences of the Ukrainian SSR. Translated from Poroshkovaya Metallurgiya, No. 6(198), pp, 55-58, June, 1979. Original article sub-

mltted July 17, 1978.

394 0038-5735/79/1806-0394507.50 �9 1979 Plenum Publishing Corporation